In this laboratory exercise, you are going to determine the history of a geologic cross sections. Download Lab_10_Dating_Rock_Strata.docx

Lab10 Exercise: Dating Rock Strata

During this lab exercise, you are going to determine the history of the geologic cross section in

the Figure X below through three methods:

(1) relative dating of the geologic features;

(2) absolute dating of the igneous and metamorphic rock layers;

(3) the use of index fossils to determine the age of sedimentary layers.

Instructions

Step 1: List each of the geologic features shown in Figure X from youngest (1) to

oldest (8). Unconformities (i.e., angular unconformity, disconformity, and

nonconformity) are indicated by a wiggly line between rock layers.

Rock units A, D, E, and F are sedimentary.

Rock units B and C are igneous.

Rock unit G is metamorphic.

Figure X

(from 1 to 9, 5 points total)

1. ______ (youngest rock unit)

2. _____

3. _____

4. _____

5. _____

6. _____

7. _____

8. _____

9. _____ (oldest rock unit)

10. What letters represents unconformities, and what type of unconformities are they? (3points)

Step 2: Calculate the ages of the metamorphic and igneous layers based on the following

information.

• A mineral sample from rock unit B has 150,000 atoms of uranium-235 and

50,000 atoms of lead-207.

• A mineral sample from rock unit C has 180,000 atoms of uranium-235 and

20,000 atoms of lead-207.

• A mineral sample from rock unit G has 100,000 atoms of potassium-40 and

100,000 atoms of argon-40.

Use Figure 9.8 (page 199), if necessary, to help you determine calculate how many half-lives have passed

since rock units B, C, and G were each formed, based on the remaining amount of a parent

isotope compared to the original amount of parent isotope.

Recall that the original amount of parent isotope can be calculated by adding together the amount

of parent isotope plus the amount of daughter isotope, making the assumption that neither has

escaped the sample. Also note that if less time has passed than a full half-life, you must calculate

what portion of a half-life has passed to make the calculations.

Once you know how many half-lives have past since the sample crystallized, use the half-life

table in Figure 9.9 (Page 200) to calculate an absolute date in mya (million years ago) for each rock unit.

(from 11 to 13, 1 point each)

11. The igneous rock in rock unit B was formed _____

12. The igneous rock in rock unit C was formed ______

13. The metamorphic rock in rock unit G was formed _____

Step 3: Index fossils have been found in the sedimentary layers! Use Figure 9.11 (page 202) to help you to identify the periods in which layers A, D, E, and F were formed as well as

their age in mya based on the discovery of the following fossils:

14. The following fossils were found in Layer A:

(2 points)

Layer A was formed ________ mya during the __________ Period.

15. The following fossils were found in Layer D:

(2 points)

Layer D was formed _________ mya during the __________ Period.

16. The following fossils were found in Layer E: (2 points)

Layer E was formed _________ mya during the __________ Period.

17. The following fossils were found in Layer F:

(2 points)

Layer F was formed approximately ________ mya during the _________ Period.

18. What is the least amount of time that could have passed during the hiatus represented by

youngest unconformity seen in Figure X? (1 point)

Lab 10 Geological Time